1. Field of the Invention
The invention in general relates to the fabrication of integrated circuits utilizing metal oxides, such as barium strontium titanate, and more particularly to the fabrication of thin film capacitors on gallium arsenide substrates.
2. Statement of the Problem
Metal oxide materials, such as barium strontium titanate, commonly referred to as BST, are known to be useful in making integrated circuit thin film capacitors having high dielectric constants. See for example, Kuniaki Koyama, et al., "A Stacked Capacitor With (Ba.sub.x Sr.sub.1-x)TiO.sub.3 For 256M DRAM" in IDEM(International Electron Devices Meeting) Technical Digest, December 1991, pp. 32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. In both of these references, the BST capacitors are fabricated on a silicon substrate. While the results were good at low frequencies, i.e., about 10 megahertz, up to now, metal oxide thin film capacitors having high capacitance at high frequencies, i.e., at frequencies of 1 gigahertz and higher, have not been possible.
It has been shown that a PZT ferroelectric RAM can be fabricated on gallium arsenide substrate, and further that silicon nitride (SIN) is effective in encapsulating the GaAs to prevent contamination of the PZT. See "Process Technology Developments For GaAs Ferroelectric Nonvolatile Memory" by L. E. Sanchez et al., and "Integrated Ferroelectrics" by J. F. Scott et al., in Condensed Matter News, Vol. 1, No. 3, 1992. This article also discloses the use of a silicon dioxide (SiO.sub.2) layer between the silicon nitride and the PZT capacitor. However, while the article indicates that a successful memory was made using the process, it also suggests that, due to problems relating to the interaction of the ferroelectric material with the GaAs, one can expect the electronic properties to be at best the same, but, more likely, less than those of a comparable device built on a silicon substrate.
It is well-known, the process of spin coating has been used for making certain types of insulators in integrated circuits, such as spin-on glass (SOG). A carboxylate-based spin-on process has also been used for making metal oxides such as barium titanate, strontium titanate, and barium strontium titanate. See G. M. Vest and S. Singaram, "Synthesis of Metallo-organic Compounds For MOD Powders and Films", Materials Research Society Symposium Proceedings, Vol. 60, 1986, pp. 35-42, Robert W. Vest and Jiejie Xu, "PbTiO.sub.3 Thin Films From Metalloorganic Precursors", IEEE Transactions On Ultrasonics, Ferroelectrics, and Frequency Control, Vol 35, No. 6, November 1988, pp. 711-717, and "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", Materials Research Society Bulletin, October 1989, pp. 48-53. However, the quality of the thin films made in these references was far too poor for use in integrated circuits, and these processes have, up to the time of the present invention, been used only for screen printing of metal oxide inks in making relatively macroscopic parts of circuits. Thus this spin-on technique did not appear to be a suitable candidate for a fabrication process which might produce state-of-the-art integrated circuit devices, such as high-capacitance, high-frequency thin film capacitors. Since the use of the GaAs substrate technology and the carboxylate spin-on technology both lead to less satisfactory results than, say, the silicon-based technology and deposition by sputtering, it would seem unlikely that their combination could lead to metal oxide thin film capacitors having high capacitance at high frequencies, i.e., at frequencies of 1 gigahertz and higher.
3. Solution to the Problem
The invention solves the problem of providing high-capacitance, high-frequency thin film capacitors by utilizing alkoxycarboxylate liquid precursors and a spin-on technique to deposit the metal oxide thin films on gallium arsenide substrates. Preferably the gallium arsenide is encapsulated by a barrier layer which prevents the volatilization of the GaAs in subsequent annealing steps at high temperature, which is in turn covered by a stress-reducing layer which lowers the stress between the gallium arsenide and the metal oxide capacitor. Preferably the barrier layer is comprised of silicon nitride (Si.sub.3 N.sub.4) and the stress-reduction layer is comprised of silicon dioxide. Preferably a relatively low temperature spin-on process as described in co-owned U.S. patent application Ser. No. 08/165,082, now abandoned, incorporated herein by reference, is used to deposit the metal oxide.
The use of a liquid precursor spin-on process to deposit the metal oxide permits much more accurate control of the stoichiometry of the metal oxide and also results in a much more homogeneous material. This homogeneity and careful control of the drying and annealing processes leads to electronic properties are much better than for thin film devices fabricated by prior art methods. Further, the homogeneity of the metal oxides significantly reduces the stresses and cracking that accompanied prior art fabrication methods. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.